Capacitor having an improved linear behavior

ABSTRACT

A capacitor having improved linear properties is provided. The capacitor is compatible with manufacturing processes of components which function using BAW. The capacitor comprises a first and a second electrode (E 1 , E 2 ) in a first electrically conductive layer and a third electrode (E 3 ) in a second electric layer. A dielectric layer (DL) is arranged between the electrically conductive layers. The first electrode (E 1 ) and the second electrode (E 2 ) are the terminal electrodes of the capacitor.

The present invention relates to capacitors having an improved linearbehavior, for example, for high-frequency (HF) circuits.

Capacitors are the real implementations of ideal capacitive elements. Innetwork theory, capacitive elements are therefore characterized solelyby their capacitance, while capacitors, in addition to their capacitivebehavior, also generally have undesirable frequency and temperaturedependencies, parasitic inductances, and a nonlinear behavior.

The occurrence of undesirable IMD (intermodulation distortion) productsis possible particularly in HF circuits, as, for example, may be used inmobile communication devices. Current cellular network systems areplacing ever-greater demands on all components in their transceivers,and thus on their capacitors as well. In particular, a nonlinearbehavior of components, which in practice results in the generation ofharmonic and intermodulation products in the signal path, may sharplyreduce the sensitivity of the receiver side.

Mobile communication devices may, for example, comprise filtercomponents which function using acoustic waves, for example, using bulkacoustic waves. Corresponding filter circuits may therefore includecapacitors whose electromagnetic properties should correspond to thoseof ideal capacitive elements.

For example, DE 10 2008 045 346 A1 describes duplexer circuits of mobilecommunication devices containing HF filters.

From DE 10 2009 011 639 A1, it is known to integrate capacitors intocomponents which function using bulk acoustic waves in order to obtain acomponent having small dimensions and good electrical properties.

The problem with known capacitors for HF filters is the aforementionedundesirable properties of real capacitors.

One object of the present invention is therefore to provide capacitorswhich are suitable for use in HF filter circuits and have an electricalbehavior which comes as close as possible to that of ideal capacitiveelements and in particular have an improved linear behavior.Furthermore, such capacitors are to cause no parasitic resonances ifthey, for example, are implemented in components which function usingbulk acoustic waves. Additionally, corresponding capacitors are to beproducible in an economical manner and without increasing themanufacturing complexity. Furthermore, a corresponding componentincluding such a capacitor is to be compatible with the continuing trendtoward miniaturization.

These objects are achieved via a capacitor according to claim 1.Dependent claims provide advantageous embodiments of the presentinvention. The features described in the claims and hereinafter mayinteract in any combination in order to produce an individually matchedcapacitor.

A capacitor comprises a first and a second electrically conductivelayer. The capacitor furthermore comprises a first and a secondelectrode structured in the first layer and a third electrode structuredin the second layer. A dielectric layer is arranged between theelectrically conductive layers. A portion of the first electrode and aportion of the second electrode each overlap with at least a portion ofthe third electrode. The first electrode and the second electrode arethe terminal electrodes of the capacitor.

The term “overlap” refers to the fact that corresponding areas of theelectrodes are arranged lying flat facing each other. At least theoverlapping parts of the electrodes each essentially implement a platecapacitor.

The capacitor is connectable to a circuit environment via the terminalelectrodes of the capacitor, i.e., via the first electrode and thesecond electrode.

It is possible that the first electrode and the third electrode togetherform a first partial capacitor, while the third electrode and the secondelectrode form a second partial capacitor. The capacitor is thenessentially the series connection of both partial capacitors.

The capacitor thus has a layer structure which is compatible withmanufacturing processes of components functioning using acoustic waves,for example, bulk acoustic waves.

It is possible to obtain good electrical properties of the capacitor viaa suitable choice of the materials of the layers. In particular, thechoice of the material of the dielectric layer has a strong influence onthe linear properties of the capacitor.

In one specific embodiment, the dielectric layer is not piezoelectric.The material of the dielectric layer thus differs substantially from thematerial of a piezoelectric layer of components which function usingbulk acoustic waves.

Components which function using bulk acoustic waves comprise apiezoelectric layer between two electrodes. A layer stack made up of alower electrode, a piezoelectric layer, and an upper electrode iselectroacoustically active if an HF signal is applied to theseelectrodes having the resonance frequency of the layer stack. In such alayer stack, an acoustic resonance then develops. Thus, anelectroacoustic resonator exists which manifests a highly nonlinearbehavior. If a capacitor is to be obtained instead of a so-called BAW(bulk acoustic wave) resonator, it is thus possible to detune acorresponding layer stack via mechanical properties such as thedeposition of layers detuning the resonator on the surface of the upperelectrode. However, an ideal capacitive element is not achieved in doingso.

There are BAW resonators in which an acoustic mirror is arranged whichsupports the development of acoustic resonances. Such a mirror maycomprise a series of layers of materials having alternately highacoustic impedance and low acoustic impedance. Materials having highacoustic impedance are in particular materials having a high specificdensity, for example, metals. The use of a capacitor whose dielectric isnot the piezoelectric layer of a BAW resonator stack, but which uses amaterial of the mirror as the dielectric, is possible, but highlycomplex, since the corresponding electrodes, in particular the lowerelectrode, are buried deep in the layer stack, and a via extendingrelatively deeply through

1. the layer having the piezoelectric material,

2. the layer having the lower electrode of the BAW resonator, and

3. the layer having the dielectric material of the capacitor arrangedbelow the lower electrode of the BAW resonator is necessary.

The present invention now provides a capacitor in which no via throughthe layer having the dielectric material beneath the layer having thepiezoelectric material is necessary. Thus, a capacitor is provided whoseelectrodes may be easily contacted. As a result of the dielectricmaterial not being piezoelectric, excellent electrical, in particularlinear, properties are obtained.

In one specific embodiment, the dielectric layer of the capacitor iscorrespondingly a mirror layer of a component which functions usingacoustic waves, for example, using bulk acoustic waves.

In one specific embodiment, the first electrically conductive layer is alayer in which electrodes of a resonator which functions using bulkacoustic waves are arranged. In particular, it is possible that thefirst electrically conductive layer comprises an electrode of aresonator which functions using bulk acoustic waves.

In one specific embodiment, the first electrode is an electrode of aresonator which functions using bulk acoustic waves. At least one of thepartial capacitors is correspondingly arranged below a BAW resonatorstack. Since the first electrode is an electrode of a BAW resonator,electrical access to the first electrode is easily possible. In such acase, another via through the layer having the piezoelectric materialwould at most be necessary in order to reach and contact the secondelectrode.

In one specific embodiment, the capacitor correspondingly comprises apiezoelectric layer above the first electrically conductive layer and anadditional electrically conductive layer above the piezoelectric layer.In the additional electric layer above the piezoelectric layer, theupper electrodes may be structured from BAW resonator stacks. If a viathrough the layer having the dielectric material above the secondelectrode is present, structured material of the additional electricallyconductive layer may thus establish a connection to the secondelectrode.

Thus, a capacitor is obtained which may be formed essentially withoutadditional manufacturing processes. In particular, no via through thedielectric material of the capacitor is necessary.

In one specific embodiment, one of the two electrodes which is selectedfrom a first and a second electrode is correspondingly electricallyconnected to an HF filter circuit by means of a via through thepiezoelectric layer.

In one specific embodiment, at least one of the two electrodes, selectedfrom a first and a second electrode, is arranged beneath a bumpconnection and is electrically connected to the bump connection.

BAW resonator stacks, in particular having acoustic mirrors, arearranged on a carrier substrate and are part of a BAW chip. Such a chiphaving a flip-chip construction may be electrically connected to andlinked with other circuit components or corresponding components (i.e.,via bump connections). In order for the bump connections not to disturbthe electroacoustic resonators, they are not arranged on the BAWresonators, but rather next to the BAW resonators. In order to obtainlow-height components, the bump connections may be arranged in recessesin the layer having the piezoelectric material. In such a case, therecess in the piezoelectric material may thus constitute a via up to thefirst or second electrode of the capacitor.

A so-called UBM (under-bump metallization) may be arranged between oneof the electrodes of the capacitor and a bump connection in order toobtain a good electrical and mechanical connection of the bumpconnection.

In one specific embodiment, the capacitor is part of an HF filtercircuit. In particular, transmission and/or reception filters ofduplexer circuits for mobile communication devices are possible asfilter circuits. Particularly great demands with respect to linearityare placed in particular on capacitors electrically connected to anantenna path, in order to keep the development of intermodulationproducts and harmonics as low as possible. The use of a capacitoraccording to the present invention near an antenna path in terms ofcircuitry is therefore especially advantageous.

It is thus possible to connect a corresponding capacitor in an impedancematching circuit of a duplexer.

In one specific embodiment, the capacitor is part of an absorptioncircuit of an HF filter circuit. Via such an absorption circuit,undesirable frequency components which would generate intermodulationproducts having particularly disturbing frequency components may bedischarged to ground.

In one specific embodiment, the capacitor is part of a rejection circuitof an HF filter circuit.

In one specific embodiment, the capacitor is electrically connected inparallel with an inductive element of an HF filter circuit. Whenmatching the impedance to inductive elements electrically connected inparallel at a filter port, small inductance values cause long lines inthe Smith chart. For the most part, due to lack of space, it is notpossible to introduce an inductive element having a larger inductance inthe housing for obtaining short lines in the Smith chart. Furthermore,there are application cases in which an inductive element electricallyconnected in series is not an option. In such a case, a correspondingcapacitor in a parallel branch may compensate for the effect of a smallparallel inductance, so that an inductive element having large geometricdimensions may correspondingly be made smaller or even be omittedentirely.

In one specific embodiment, the capacitor is part of a duplexer matchingcircuit including a π-element or including a T-element.

π-elements and T-elements are highly suitable for impedance matching,for example, between a transmission filter and a reception filter of aduplexer, since such duplexer matching circuits are generallyelectrically connected directly to the antenna path.

In one specific embodiment, the capacitor is coupled directly, orindirectly via other circuit elements, to a resonator which functionsusing acoustic waves which is not arranged directly adjacent, theresonator which functions using bulk acoustic waves resonator not beingarranged directly adjacent to the capacitor. A connection of acorresponding capacitor to a BAW resonator is thus also possible if toolittle space is available in the immediate vicinity of the BAW resonatoror if acoustic waves emitted by the resonator would disturb thefunctioning of the capacitor.

In one specific embodiment, the capacitor comprises a series circuitmade up of two, four, six, eight, ten, or more partial capacitors. Inthis case, it is not necessary for the third electrode to overlap withboth the first electrode and the second electrode. It is possible thatthe first electrode overlaps with an electrode of the secondelectrically conductive layer. This electrode of the second electricallyconductive layer overlaps with another electrode of the firstelectrically conductive layer, which in turn overlaps with anotherelectrode of the second electrically conductive layer. Thus, aninterlinking of partial capacitors from the first electrode to thesecond electrode via a corresponding number of intermediate electrodesis obtained.

In one specific embodiment of the capacitor, the dielectric layer islocally thinned in the area of at least one electrode. The thickness ofthe dielectric layer may be reduced in comparison to the thickness ofthe dielectric material of the corresponding mirror layer of a BAWcapacitor.

The materials of the first and second electrically conductive layers andthe dielectric layer may comprise the materials of conventional BAWresonator stacks. Thus, aluminum, copper, gold, and silver as metalshaving high electrical conductivity, or alloys made up of these metals,are possible. The electrically conductive layers may also comprisetitanium, for example, as an adhesion-promoting layer. In particular, itis possible that the electrically conductive layers in turn arethemselves made up of a plurality of layers including various metals oralloys.

The dielectric layer may in particular comprise nonconductive materialshaving high or low acoustic impedance, for example SiO₂, Si₃N₄ and/orTa₂O₅. The dielectric layer may also comprise multiple partial layers.

The inverse capacitance of a series connection of partial capacitors isessentially the sum of the inverse capacitances of the correspondingpartial capacitors. In order to obtain a capacitance per required areawhich is as large as possible, the partial capacitors i.e., theiroverlap areas, are essentially equal in size.

Furthermore, the capacitance of the partial capacitors is a function ofthe spacing of the electrodes and the dielectric constant of thedielectric. Correspondingly, the dielectric material may have aparticularly high or particularly low dielectric constant, and thethickness of the dielectric layer may be set to a predefined value, forexample, via thinning.

In one specific embodiment, the capacitor is electrically connected aspart of a duplexer circuit. In the duplexer, in addition to thepreviously described capacitor, additional capacitors may be obtainedhaving a similar construction. It is also possible for all capacitors ofthe duplexer to be constructed as described above.

The capacitor may in particular be electrically connected in an antennamatching circuit of the duplexer, for example, as a capacitive elementof a π circuit having two inductive elements and the capacitive serieselement between them.

The antenna matching circuit may constitute a phase shifter in theduplexer circuit. Furthermore, the entire phase shifter may be designedin an IPD (integrated passive device) technology and thus implemented ina space-saving manner in the duplexer.

The duplexer may be a hybrid duplexer including BAW resonators in the Txsignal path and including SAW resonators in the Rx signal path.

It is furthermore possible that the thickness of the dielectric layer inthe area of the first and the second electrode is set in such a way thata virtual resonance frequency is shifted to higher or lower frequenciesby more than the duplexer spacing. The virtual resonance frequency isthat frequency which would be the acoustic resonance frequency of thecapacitor if the dielectric layer were piezoelectric.

The capacitor is described in greater detail below based on exemplaryembodiments, which are to be understood as nonrestrictive, andassociated schematic figures.

FIG. 1 shows the essential arrangement of a first, second, and thirdelectrode,

FIG. 2 shows the arrangement of the electrodes in a layer stack,

FIG. 3 shows the use of the capacitor in a BAW resonator stack,

FIG. 4 shows one specific embodiment in which the dielectric layer has adifferent thickness in different partial capacitors,

FIG. 5 shows one specific embodiment in which the terminal electrodesare electrically connected by means of vias,

FIG. 6 shows the equivalent circuit diagram of the capacitor,

FIG. 7 shows one specific embodiment in which the capacitor iselectrically connected in parallel with a BAW resonator,

FIG. 8 shows the equivalent circuit diagram of the electrode structureshown in FIG. 7,

FIG. 9 shows one specific embodiment of a parallel connection of thecapacitor to a BAW resonator having three terminals,

FIG. 10 shows the equivalent circuit diagram of the electrode structureof FIG. 9,

FIG. 11 shows one specific embodiment in which one of the electrodes isthe extension of a lower electrode of a BAW resonator,

FIG. 12 shows the equivalent circuit diagram of the specific embodimentof FIG. 11,

FIG. 13 shows one specific embodiment of the capacitor having sixpartial capacitors,

FIG. 14 shows the equivalent circuit diagram of a series connection ofthe capacitor to a resonator,

FIG. 15 shows one specific embodiment of the capacitor in which thesecond electrode is arranged beneath a bump connection,

FIG. 16 shows the equivalent circuit diagram of a filter circuit inwhich the capacitor is electrically connected in parallel with aresonator,

FIG. 17 shows the equivalent circuit diagram of a filter circuit inwhich the capacitor is electrically connected in a rejection circuit,

FIG. 18 shows the equivalent circuit diagram of a filter in which thecapacitor is electrically connected in parallel with an inductiveelement L2 and in parallel with a resonator X,

FIG. 19 shows the equivalent circuit diagram of a duplexer circuit inwhich the capacitor is electrically connected in a π-element,

FIG. 20 shows the equivalent circuit diagram of a duplexer in which thecapacitor is electrically connected in a T-element,

FIG. 21 shows a filter circuit with the capacitor as a series branchelement,

FIG. 22 shows a filter circuit in which the capacitor is electricallycoupled to a resonator which is arranged at any location,

FIG. 23 shows a duplexer circuit including the capacitor in a matchingcircuit.

FIG. 1 schematically depicts the arrangement of the three electrodes ofthe capacitor. A first electrode E1 and a second electrode E2 arestructured in a first electrically conductive layer EL1. A thirdelectrode E3 is structured in a second electrically conductive layerEL2. A dielectric layer DL is arranged between the first electricallyconductive layer EL1 and the second electrically conductive layer EL2.At least a portion of the first electrode E1 overlaps with a portion ofthe third electrode E3. At least a portion of the second electrode E2likewise overlaps with an additional portion of the third electrode E3.The overlap area of the first electrode E1 and the third electrode E3essentially forms a plate capacitor, symbolized by the capacitiveelement C1. The overlap area of the second electrode E2 with thecorresponding area of the third electrode E3 likewise forms a platecapacitor, symbolized/depicted by the capacitive element C2. Thecapacitor of FIG. 1 is thus essentially a series connection of twopartial capacitors. The electrodes E1, E2 are the terminal electrodes ofthe capacitor, via which the capacitor may be electrically connected toa circuit environment. Correspondingly, the first electrode E1constitutes a first terminal T1, while the second electrode E2constitutes a second terminal T2. The capacitor thus provides a totalcapacitance which is a function of the partial capacitances C1, C2,which it is possible to access without a via through the dielectriclayer DL.

The material or the materials of the dielectric layer DL and thematerials of the electrodes may be selected in such a way that thecapacitor has sufficiently good electrical properties even at criticalfrequencies.

FIG. 2 shows one specific embodiment of the capacitor in which thedielectric layer DL and the third electrode E3 are part of a mirror MIRof a BAW resonator of the mirror type. The mirror MIR is arranged on asubstrate SU. The depiction of the mirror MIR is symbolic, i.e., themirror MIR may comprise additional mirror layers having alternately highand low acoustic impedance.

The capacitance of the capacitor is a function of the overlap areas ofthe electrodes. The overlap area of the electrodes is in particular afunction of the electrode area or its shape and diameter w1, w2. Byselecting suitable overlap areas, the desired total capacitance of thecapacitor may be set, where the following applies: 1/C_(tot)=1/C1+1/C2.C_(tot) is the total capacitance of the capacitor, while C1 and C2 arethe corresponding capacitances of the partial capacitors.

A dielectric material and an additional electrode material which isarranged on it may be present above the first electrically conductivelayer EL1 for upper electrodes of the resonator stack.

Thus, the upper nonconductive mirror layer of a BAW resonator may beused as a dielectric for the capacitor. The bottom electrode maycomprise multiple single layers. The electrical contacting of thecapacitor is carried out via the two electrodes E1, E2 as electricterminals. The materials below the mirror are not dependent on thecapacitor and may therefore be arbitrary. For manufacturing reasons, apiezoelectric layer which is possibly present above the firstelectrically conductive layer EL1 may cover an entire wafer in which thecomponent of the capacitor is manufactured and may have recesses, forexample, through-etchings, at only a few locations.

FIG. 3 shows one specific embodiment of the capacitor by way of example,in which a piezoelectric layer PZ is arranged above the firstelectrically conductive layer EL1 with the electrodes E1, E2. In orderto obtain a BAW resonator, an additional upper electrode TE is necessaryabove an electrode arranged beneath the piezoelectric layer, here, thefirst electrode E1 as the lower electrode BE. If an HF signal is presentat the electrodes TE, BE, a standing acoustic wave may develop in thelayer stack. The presence of a piezoelectric layer PZ is necessary fordeveloping the acoustic wave, i.e., for the functional capability of theBAW resonator. However, such a piezoelectric layer as a dielectric inthe capacitor would result in undesirable nonlinear effects. Thisarrangement of the dielectric layers and electrode layers relative toeach other thus provides a component in which a BAW resonator may becombined and electrically connected to a capacitor which functions in ahighly linear manner.

FIG. 4 shows one specific embodiment of the capacitor in which thespacing of the overlapping electrode areas, i.e., the thickness of thedielectric of the two partial capacitors T1, T2, is chosen differently.The resulting capacitance of a partial capacitor is:C=∈ ₀∈_(r) A/D.

A is the overlapping area and ∈_(r) is the dielectric constant of thedielectric layer, for example, the upper light-conducting mirror layer.The total capacitance of the partial capacitor may be easily set bysetting the distance D. The thickness may be reduced to a thicknessd1>d2, in particular in an area of the capacitor, for example, viathinning. Thus, the area-specific capacitance of partial capacitors maybe increased at least locally.

FIG. 5 shows one specific embodiment of a capacitor in a BAW component,the electrodes E1, E2 having been made accessible using vias through apiezoelectric layer PZ for connecting to additional circuit components.The terminals T1, T2 to the capacitor are thus provided via themetallizations ME in the vias VIA.

The vias VIA through the piezoelectric layer PZ may, for example, becreated by etching. It is also possible that the electrodes E1, E2 arecreated directly by the metallization ME arranged in the vias VIA. Themetallizations ME then touch the dielectric layer directly.

A bump connection as shown by way of example, for example, in FIG. 13,may be arranged and electrically connected in the recess of the via.

FIG. 6 shows the equivalent circuit diagram of the capacitor, i.e., aseries connection of two partial capacitors between the terminals T1 andT2. The figure thus shows the equivalent circuit diagram of thecapacitor from FIG. 5.

FIG. 7 shows one specific embodiment of the capacitor in which the firstelectrode E1 is simultaneously the lower electrode of an additionallayer stack comprising a dielectric material D2 and a metallization ofthe terminal T2. If the additional dielectric material D2 ispiezoelectric, the first electrode E1 is simultaneously the lowerelectrode of the corresponding BAW resonator stack. The electrodearrangement of FIG. 7 shows a parallel connection of the capacitor to aresonator comprising the dielectric layer D2. FIG. 7 thus depicts aspace-saving specific embodiment of a parallel connection of a capacitorto a resonator. Such an electrode arrangement in particular makespossible the targeted reduction of the resonator coupling of the BAWresonator BAW.

Such an arrangement makes it possible to save chip area, because thepartial capacitor comprising the electrodes E1 and E3 is arrangeddirectly beneath the resonator BAW. The space beneath the resonator BAWis in fact generally not available for additional BAW resonator stacksbecause these areas are acoustically coupled to each other.

FIG. 8 shows the equivalent circuit diagram of the electrode structurefrom FIG. 7.

FIG. 9 shows one specific embodiment in which, unlike the specificembodiment of FIG. 7, the second electrode E2 of the capacitor is notelectrically connected to the upper electrode of the BAW resonator BAW.The specific embodiment of FIG. 9 thus constitutes a component havingthree independent terminals T1, T2, T3.

It is possible that the area of the second electrode E2 exceeds therequired area of the terminal T2 by a factor x. x may lie between 1 and100 and may be in particular 1.1, 2, 5, 10, and 50.

FIG. 10 shows the equivalent circuit diagram of the electrodearrangement of FIG. 9.

FIG. 11 shows one specific embodiment in which the first electrode E1 ofthe capacitor is obtained as an extension of the lower electrode of aBAW resonator. The BAW resonator comprises a part of the electricallyconductive layer including the first electrode E1, material of thepiezoelectric layer, and material of an electrode layer on thepiezoelectric layer.

FIG. 12 shows the equivalent circuit diagram of the electrodearrangement of FIG. 11. A terminal T3 is electrically connected to theresonator, but not directly to the capacitor. A terminal T2 iselectrically connected to the capacitor, but not directly to theresonator. A terminal T1 is electrically connected directly both to theresonator and the capacitor.

FIG. 13 shows one specific embodiment of the capacitor by way of exampleincluding six partial capacitors. In addition to the first electrode E1and the second electrode E2, the first electrically conductive layer EL1comprises additional electrode structures E. The second electricallyconductive layer EL2 correspondingly comprises additional structuredelectrode sections, so that a corresponding series connection isobtained from six partial capacitors. In this way, a series connectionmade up of 2n partial capacitors may be easily obtained, where n is anatural number >1.

FIG. 14 shows the equivalent circuit diagram of a possible connection ofthe capacitor, i.e., a series connection, to a resonator.

FIG. 15 shows one specific embodiment of a capacitor, in which thecontacting of the second electrode is implemented via a bump connectionBU. The bump connection BU constitutes the second terminal of thecapacitor. The bump connection BU may sit directly on the secondelectrode. It is also possible for the bump connection BU to be arrangedon a metallization ME in a recess through the piezoelectric layer PZ. Anadditional layer, for example, a so-called UBM (under-bumpmetallization) may be arranged between the bump connection BU and thefirst electrode E1 or the metallization ME for improving contact betweenthe bump BU and the electrode.

The implementation of the bump connection BU may be carried out via apiece of a bonding wire, via solder, or other conventional manufacturingmethods. Since a bump connection advantageously includes a bottomstructure, it is recommended to situate one of the electrodes E1, E2under the bump connection and to move the corresponding section of theuppermost conductive mirror layer under the bump connection as a thirdelectrode E3, in order to obtain a maximum overlapping structure andthus maximum capacitance of the capacitor.

The third electrode E3 comprises an area which is arranged under thebump connection.

FIG. 16 shows an equivalent circuit diagram of a filter circuit F inwhich a capacitor comprising the two partial capacitors C1, C2electrically connected in series is electrically connected in parallelwith a resonator X as a parallel branch element, for example, in aladder-type filter structure. The capacitor may thus connect a terminalB which is, for example, electrically connected to a signal path, to aterminal A which is, for example, electrically connected to ground.

An inductive element L for connecting to ground may also be electricallyconnected between the terminal A and a ground terminal. The directconnection to the resonator X is not necessary. The capacitor may alsoconnect the terminal B directly to the terminal A without additionalcircuit elements. More than one capacitor according to the presentinvention may be included in a filter circuit F and in a correspondingcomponent; the dots on the left FIG. 16 correspondingly indicate thatthe depicted circuit elements constitute only a portion of a moreextensive circuit.

FIG. 17 shows a capacitor including the partial capacitors C1 and C2which is electrically connected in a rejection circuit. The capacitor iselectrically connected to the inductive elements L1 and L2. Inparticular, the capacitor is electrically connected in series betweenthe two inductive elements L1, L2. FIG. 17 depicts the capacitor as partof a rejection circuit, for example, at a filter port FP. The resonancefrequency f of the rejection circuit is

$f = {\frac{1}{2\pi\sqrt{\left( {L_{1} + L_{2}} \right)\left( {C_{ges} + C_{s}} \right)}}.}$C_(x) is the static capacitance of the resonator X, and C_(tot) is1/(1/C₁+1/C₂). The inductive elements L1, L2 are optional and may beomitted as required or implemented as parasitic inductances, forexample, of feed lines. The parallel connection to the resonator X isoptional. Without the connection to the resonator X, a correspondinglycorrected blocking frequency results.

FIG. 18 shows the equivalent circuit diagram of a filter circuit F, thecapacitor including the partial capacitors C1, C2 being electricallyconnected in parallel with an inductive element L2. The inductiveelement L2 may be part of an impedance matching circuit between thefilter circuit F, for example, a transmission or reception filter, andan additional filter, for example, a corresponding reception ortransmission filter of a duplexer. When matching impedance usingparallel inductive elements and a filter port, small inductance valuescause long lines in the Smith chart. Due to the lack of space in thecorresponding component, it is usually not possible to introduce aninductive element having a higher inductance in a corresponding housing.There are also application cases in which a serial inductance forcompensation is not possible. The capacitor having good linearproperties then provides relief if it is electrically connected inparallel with the corresponding inductive element L2. The specificembodiment of FIG. 18 depicts yet another inductive element L1 which,however, is optional and may be omitted, or which may be implemented viaa parasitic inductance, for example, a feed line. The connection of thecapacitor to the resonator X is also optional and is not mandatory.

FIG. 19 shows the equivalent circuit diagram of a duplexer circuitincluding two filter circuits F. The capacitor including the partialcapacitors C1 and C2 is part of a π-element circuit, for example, forimpedance matching in the antenna port of the duplexer. More precisely,the capacitor is electrically connected in a parallel path of theπ-element. An additional capacitive element C is electrically connectedin the other parallel path of the π-element. The additional capacitiveelement C may also be a capacitive element according to the presentinvention or a conventionally manufactured capacitive element.

FIG. 20 shows the equivalent circuit diagram of a duplexer circuitincluding two filter circuits F, the capacitor including the partialcapacitors C1, C2 being electrically connected in the series path of aT-element matching circuit. An inductive element L is electricallyconnected in the parallel branch of the T-element. The inductive elementaccording to the present invention is electrically connected in serieswith a capacitive element C which is conventional or which is alsoaccording to the present invention.

FIG. 21 depicts a general equivalent circuit diagram in which thecapacitor is electrically connected as a series branch element to anyarbitrary additional circuit components.

FIG. 22 symbolizes a circuit arrangement in which the capacitorincluding the partial capacitors C1, C2 is coupled directly orindirectly to a resonator which is arranged at any location. A spatialproximity between the capacitor and the resonator is not mandatory inorder to achieve the advantages of the capacitor. The parallel circuitmade up of the resonator and the capacitor may be electrically connectedto ground via an inductive element L.

FIG. 23 depicts a duplexer circuit DU having an antenna terminal ANTbetween a transmission terminal Tx and a reception terminal Rx. Amatching circuit including two inductive elements IE and the capacitorincluding the two partial capacitors C1, C2 is electrically connected tothe antenna terminal ANT.

The transmission signal branch functions essentially using bulk acousticwaves, while the reception branch functions using acoustic surfacewaves. FIG. 23 thus constitutes a hybrid duplexer. A ground element of aladder-type structure having a triple cascade in the signal path andquadruple cascade in the parallel branch path is electrically connectedbetween the antenna terminal ANT and the reception terminal Rx. A DMS(dual mode SAW) structure is electrically connected between the groundelement and the terminal Rx. The reception terminal Rx is balanced. Thetransmission terminal is unbalanced.

A capacitor according to the present invention is not limited to one ofthe described exemplary embodiments. Combinations of features of theexemplary embodiments and variations which, for example, compriseadditional resonators, inductive or capacitive elements, or partialcapacitors, also constitute exemplary embodiments according to thepresent invention.

LIST OF REFERENCE NUMBERS

-   A, B: Filter terminals-   ANT: Antenna terminal-   BAW: BAW resonator stack, BAW resonator-   BE: Lower electrode of a BAW resonator stack-   BU: Bump connection-   C: Capacitive element-   C1, C2: First, second partial capacitor-   D1, D2: Thicknesses of the dielectric layer-   D2: Additional dielectric material-   DL: Dielectric layer-   DMS: DMS structure-   DU: Duplexer-   E: Additional structured electrodes in the first electrically    conductive layer-   E1, E2: First, second electrode-   E3: Third electrode-   EL1: First electrically conductive layer-   EL2: Second electrically conductive layer-   F: Filter-   f: Blocking frequency-   IE: Inductive element-   L1, L2: Inductive element-   ME: Metallization-   MIR: Mirror-   PZ: Piezoelectric layer-   Rx: Reception terminal-   SU: Substrate-   T1, T2: First, second terminal of the capacitor-   TE: Upper electrode of a BAW resonator stack-   Tx: Transmission terminal-   UBM: Under-bump metallization-   VIA: Via-   W1, W2: Lateral dimensions of the first, second electrode-   X: Resonator

The invention claimed is:
 1. An apparatus comprising: a bulk acousticwave (BAW) resonator stack comprising a first electrode, a second upperelectrode, a lower electrode, and a piezoelectric layer disposed betweenthe first upper electrode and the lower electrode; a mirror structuredisposed below the lower electrode of the BAW resonator stack andcomprising a dielectric layer, wherein the dielectric layer is notpiezoelectric; and a capacitor electrically connected to the BAWresonator stack and comprising: first and second electrically conductivelayers, wherein the dielectric layer of the mirror structure is disposedbetween the first and second electrically conductive layers; a firstelectrode and a second electrode disposed in the first electricallyconductive layer; and a third electrode disposed in the secondelectrically conductive layer, wherein: a portion of the first electrodeand a portion of the second electrode each overlap with at least aportion of the third electrode; the first electrode and the secondelectrode are terminal electrodes of the capacitor; the first electrodeand the lower electrode are the same electrode; and the second electrodeof the capacitor is connected to the second upper electrode through thepiezoelectric layer.
 2. The apparatus of claim 1, wherein the secondupper electrode is electrically connected to the second electrode by avia through the piezoelectric layer, such that the second upperelectrode and the lower electrode of the BAW resonator stack are theterminal electrodes of the capacitor.
 3. The apparatus of claim 1,wherein the first upper electrode is electrically disconnected from thesecond electrode and wherein the first upper electrode, the firstelectrode, and the second electrode comprise at least three terminalelectrodes for the apparatus.
 4. The apparatus of claim 3, wherein thefirst upper electrode and the first electrode have a same length.
 5. Theapparatus of claim 2, wherein the second electrode of the capacitor iselectrically connected to a high frequency (HF) filter circuit by thevia through the piezoelectric layer.
 6. The apparatus of claim 2,wherein the second electrode is electrically connected to a bumpconnection disposed above a metallization for the via through thepiezoelectric layer.
 7. The apparatus of claim 1, wherein the dielectriclayer is locally thinned in an area associated with the first electrodeor with the second electrode.
 8. A high frequency (HF) filter circuitcomprising the apparatus of claim
 1. 9. The HF filter circuit of claim8, further comprising a rejection circuit or an absorption circuit,wherein the rejection circuit or the absorption circuit comprises theapparatus of claim
 1. 10. The HF filter circuit of claim 8, furthercomprising an inductive element electrically connected in parallel withthe capacitor of claim
 1. 11. A duplexer matching circuit including api-element or a T-element, wherein the duplexer matching circuitcomprises the apparatus of claim
 1. 12. A duplexer circuit comprisingthe apparatus of claim 1.